Method for treating exhaust gas containing soot particles

ABSTRACT

A method for converting soot particles of an exhaust gas includes providing at least nitrogen dioxide or oxygen in the exhaust gas, ionizing soot particles with an electric field, depositing electrically charged soot particles on inner channel walls of at least one surface precipitator, and bringing at least nitrogen dioxide or oxygen into contact with the deposited soot particles on the inner channel walls of the at least one surface precipitator. A device for carrying out the method includes at least one surface precipitator having a plurality of channels through which the exhaust gas can flow and extending between an inlet region and an outlet region, and at least one deposit inhibitor for electrically charged soot particles provided in at least part of the inlet region, especially allowing the soot particles to be evenly deposited and the surface precipitator to be continuously regenerated.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of U.S. patent application Ser. No.13/419,812, filed Mar. 14, 2012; which was a continuation application,under 35 U.S.C. §120, of International application PCT/EP2010/062805,filed Sep. 1, 2010; the application also claims the priority, under 35U.S.C. §119, of German patent application No. DE 10 2009 041 090.2,filed Sep. 14, 2009; the prior applications are herewith incorporated byreference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for treating exhaust gascontaining soot particles, in particular with a so-called electrostaticfilter or electric filter, as well as a suitable method for convertingsoot particles of an exhaust gas. The invention is used, in particular,in the treatment of exhaust gases of mobile internal combustion enginesin the field of automobiles, in particular in the treatment of exhaustgases resulting from diesel fuel.

A multiplicity of different concepts for eliminating soot particles fromexhaust gases of mobile internal combustion engines have already beendiscussed. In addition to wall-flow filters which are alternatelyclosed, open secondary flow filters and gravity precipitators, etc.,systems have already been proposed in which the particles in the exhaustgas are charged electrically and then deposited by using electrostaticattraction forces. Those systems are known, in particular, by the term“electrostatic filter” or “electric filter.”

Generally (a plurality of) discharge electrodes and collectorelectrodes, positioned in the exhaust line, are proposed for suchelectric filters. In that context, for example, a central dischargeelectrode which runs approximately centrally through the exhaust lineand a surrounding lateral surface of the exhaust line as a collectorelectrode are used to form a capacitor. Through the use of thatconfiguration of the discharge electrode and the collector electrode, anelectrical field is formed transversely with respect to the direction offlow of the exhaust gas, wherein the discharge electrode can beoperated, for example, with a high voltage which is in the range ofapproximately 15 kV. As a result, in particular corona discharges can beformed through which the particles which flow through the electricalfield with the exhaust gas are charged in a unipolar fashion. As aresult of that charging, the particles migrate to the collectorelectrode due to electrostatic Coulomb forces.

In addition to systems in which the exhaust line is embodied as acollector electrode, systems are also known in which the collectorelectrode is embodied, for example, as a wire mesh. In that context, theaccumulation of particles on the wire mesh serves the purpose ofcombining the particles, where appropriate, with further particles inorder to therefore form an agglomeration. The exhaust gas which flowsthrough the mesh or grid then carries the relatively large particlesalong with it again and carries them to conventional filter systems.

Even if the systems described above have heretofore proven suitable forthe treatment of soot particles, at least in trials, the implementationof that concept for series production in motor vehicles presents seriouschallenges. That applies, in particular, with respect to the greatlyfluctuating, at times very heavy, soot load in the exhaust gas, as wellas the desired retrofitability of such a system for currently existingexhaust systems. In addition, it is necessary to take into account thefact that the improved performance of such exhaust systems in terms ofthe elimination of soot particles also makes it necessary to perform(periodic or continuous) regeneration of the filter systems, involvingthe soot being converted into gaseous components.

With respect to the regeneration of filter systems, it is also known, inaddition to the intermittent regeneration by brief heating, that is tosay burning off of the soot (catalytically motivated oxidativeconversion), to convert soot through the use of nitrogen dioxide (NO₂).The advantage of continuous regeneration with nitrogen dioxide is thatthe conversion of soot can already take place in that case atsignificantly lower temperatures (in particular lower than 250° C.). Forthat reason, continuous regeneration is preferred in many applications.However, that leads to the problem that it is necessary to ensure thatthe nitrogen dioxide in the stream of exhaust gas comes into contact toa sufficient degree with the deposited soot particles.

In that context, there are also technical difficulties in theimplementation of continuous operation of such exhaust systems in motorvehicles, wherein the different loadings of the internal combustionengines lead to different streams of exhaust gas, compositions ofexhaust gas and temperatures.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a device and amethod for treating exhaust gas containing soot particles, whichovercome the hereinafore-mentioned disadvantages and at least partiallysolve the highlighted problems of the heretofore-known devices andmethods of this general type. In particular, the intention is todescribe a device for treating exhaust gas containing soot particles,which similarly makes a large precipitation effect available for sootparticles and which can be satisfactorily regenerated. The intention isalso to specify a corresponding method for converting soot particles ofan exhaust gas. The device and the method are to be easily integratedinto existing mobile exhaust systems and are to be equally capable ofbeing manufactured cost-effectively.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a device for treating exhaust gascontaining soot particles. The device comprises at least one source ofnitrogen dioxide or oxygen, at least one ionization element for ionizingsoot particles, at least one neutralization element for neutralizingelectrically charged soot particles, at least one surface precipitatorhaving an inlet region, an outlet region and a plurality of channelsthrough which the exhaust gas can flow, the channels extending betweenthe inlet region and the outlet region, and at least one depositioninhibitor disposed at least partially at the inlet region for inhibitingdeposition of electrically charged soot particles.

The device proposed herein may, in particular, be part of an exhaustsystem of a motor vehicle which has a diesel engine. However, the devicecan also be made available as a modular kit for an exhaust system.

Accordingly, firstly a nitrogen dioxide source is provided. Such anitrogen dioxide source is, for example, a catalytic converter whichassists (together with other components of the exhaust gas, inparticular oxygen) the conversion of nitrogen oxides (in particularnitrogen monoxide NO) contained in the exhaust gas into nitrogendioxide. Basically, a plurality of such nitrogen dioxide sources mayalso be present, but this is not absolutely necessary. The nitrogendioxide source can usually be implemented with a catalytic converterwhich has a honeycomb body with a coating, wherein the coating hasplatinum, rhodium, palladium or the like. The nitrogen dioxide source istherefore connected downstream of the internal combustion engine, and istherefore located at least partially in the exhaust system.

While the nitrogen dioxide source is usually preferably used inrelatively “cold exhaust systems” (for example diesel engineapplications), an oxidative conversion of the soot particles with oxygenfrom an oxygen source can also be carried out at relatively hightemperatures (for example gasoline engine applications). For example,the internal combustion engine itself or a so-called secondary airinput, that is to say, in particular, the feeding in of anoxygen-containing gas into the exhaust line, is preferably considered asan oxygen source. If appropriate, chemical conversion with a catalystcan also generate oxygen, so that this can also be considered as anoxygen source.

In particular, a device which alternatively has at least one nitrogendioxide source or at least one oxygen source upstream of the surfaceprecipitator in the exhaust line is preferred.

Furthermore, at least one ionization element for ionizing soot particlesis provided. It is preferred in this case that the exhaust gas firstlyreaches the nitrogen dioxide source before it reaches the section of thedevice with the at least one ionization element. The ionization elementpreferably includes an ionization electrode or a multiplicity ofionization electrodes. The at least one ionization element is connectedto a voltage source, in particular to a high voltage source. It is alsopossible to regulate the voltage through the use of a control unit.Basically, a direct-current voltage source or an alternating-currentvoltage source can be made available.

Furthermore, at least one neutralization element for neutralizingelectrically charged soot particles is provided. The at least oneneutralization element carries out at least the task of feedingelectrical charge to the electrically charged soot particles ordischarging it therefrom (depending on the electrical charge), as aresult of which, when contact occurs with the electrically charged sootparticles, electrical neutralization or de-ionization of the sootparticles takes place. The number of ionization elements andneutralization elements basically does not have to correspond, but itmay be expedient that they do.

An electrical field is usually formed between the at least oneionization element and the at least one neutralization element. Thiselectrical field extends, in particular, in the direction of the exhaustsystem and/or in the direction of flow of the exhaust gas, wherein theexhaust gas firstly reaches the at least one ionization element andlater the at least one neutralization element. As a result, the at leastone ionization element and the at least one neutralization element areoffset with respect to one another in the direction of flow of theexhaust gas, in particular at a distance of several centimeters such as,for example, at least 5 cm, at least 15 cm or even at least 30 cm.

Furthermore, at least one surface precipitator is provided. The surfaceprecipitator is distinguished by having a plurality of channels throughwhich the exhaust gas flows, and by the channels extending between aninlet region and an outlet region. Basically, it would be possible toform a surface precipitator which only has two channels, but anembodiment in which a plurality of channels are provided, for example atleast 30, at least 50 or even at least 100 channels, is preferred. Theterm “surface precipitator” is intended to express the fact that asurface which is very large (in particular also in relation to itsvolume) is made available for the accumulation of soot particles. Incontrast to known variants, in which, where possible, the soot particleswere agglomerated one on top of the other in a tightly limited space,the objective in this case is to distribute the soot particles over alarge area over the surfaces of the channel walls which are formed bythe channels. However, this does not rule out the possibility of thesoot particles being deposited, for example, also in the interior of aporous channel wall. In particular, the external and internal surfacesof the channel walls can therefore in this case be considered to besurfaces which are suitable for the depositing of soot. A “channel” isunderstood herein to be, in particular, a delimited flow path having anextent which is clearly longer than its diameter, wherein the diameteris, in particular, significantly greater than the customary sizes of thesoot particles. Even if it is sufficient for some purposes to formseparate and discrete channels, communicating channels can neverthelessalso be made available in which an exchange of partial streams ofexhaust gas (for example through openings in the channel walls) is madepossible. Providing channels with an extent of, for example, at least 5cm, but preferably even at least 10 cm, which are, in particular,relatively small in cross section, readily permits the individualchannels or surface precipitator to be configured in a way which isadapted to the flow profile.

However, in order to ensure that in fact there is no accumulation ofsoot particles in just one plane perpendicularly with respect to thedirection of flow (as in the case of a grid or screen), at least onedeposition inhibitor for electrically charged soot particles is providedat least partially at the inlet region. This inhibitor carries out thefunction of preventing deposition (exclusive or predominant) of theelectrically charged soot particles in the inlet region. In thiscontext, the deposition inhibitor can be configured in such a way thatit is configured only for some of the channels or configured differentlyfor the channels. The deposition inhibitor can, on one hand, relate tothe surface precipitator itself, but it is also possible for the atleast one deposition inhibitor to act on the stream of exhaust gas andtherefore bring about a changed routing of the stream of exhaust gasthrough the surface precipitator. For this purpose, the at least onedeposition inhibitor can be formed in a slightly offset fashion directlyat the inlet region and/or starting from the inlet region in thedirection of the outlet region. Under certain circumstances it is alsopossible for such a deposition inhibitor also to be provided for atleast some of the channels from the inlet region to the outlet region.The configuration and/or distribution of the at least one depositioninhibitor for electrically charged soot particles in the surfaceinhibitor is to be selected in such a way that soot particles aredeposited as uniformly as possible on the channel walls of themultiplicity of channels (and not only on the front face). This leads,in particular, to a situation in which the deposited soot particles aredisposed over a large surface, that is to say at a relatively largedistance from one another, even in the case of a briefly increased sootload of the exhaust gas. This provides, in a particular way, thepossibility of regenerating the soot particles there continuously withnitrogen dioxide. This is promoted by the fact that the exhaust gas,which is conducted through the channels over a relatively long flow pathand at a short distance from the channel wall, has the possibility ofbringing about a conversion with small particles which are relativelyfree there. This results in a situation in which, in particular, anundesired drop of pressure over the surface inhibitors is avoided as theload with soot particles increases, since particularly effectivecontinuous regeneration of the surface precipitator is carried out.

In accordance with another preferred feature of the invention, the atleast one neutralization element is formed in the vicinity of the outletregion of the at least one surface precipitator. This is intended tomean, in particular, that the at least one neutralization element formspart of the outlet region, wherein it may be, in particular, part of thesurface precipitator, may be positioned in (electrical) contacttherewith or else may be disposed downstream thereof in the direction offlow. The at least one neutralization element can accordingly beaccommodated in the surface precipitator, but a separate, downstreamrefinement of the at least one neutralization element is also possible.If the at least one neutralization element is an integral component ofthe surface precipitator, the surface precipitator simultaneouslyperforms the function of neutralization, with the result that ultimatelyelectrically neutral soot particles are deposited on the surfaces of thesurface precipitator. If the at least one neutralization element can beformed downstream of the surface precipitator, the latter can beembodied in the manner of a conventional collector electrode in order toform the electrical field through the surface precipitator. The channelsare then configured, in particular, in such a way that the electricallycharged soot particles impinge on the channel wall due to the electricalforces.

It is preferred that the ionization element be disposed upstream of thesurface precipitator in the direction of flow of the exhaust gas, as aresult of which ionization of the soot particles occurs upstream of thesurface precipitator. It is particularly preferred if the ionizationelement is disposed at a distance of at least 5 cm, in particular of atleast 10 cm, upstream of the surface precipitator. The surfaceprecipitator therefore does not serve to perform ionization but ratherbasically only as a collector for the soot particles which are alreadyionized (upstream).

In accordance with a further particularly preferred feature of theinvention, the at least one surface precipitator is embodied as ahoneycomb body. Such honeycomb bodies can basically be constructed withdifferent materials, in particular also with metallic and/or ceramiccomponents. The manufacturing methods for such honeycomb bodies havebeen known for many years and the honeycomb bodies have provenparticularly suitable in terms of formation of contact between thestream of exhaust gas and the walls in the channels. In this case, thehoneycomb body can have (only) open and/or (partially) closed channels.Basically, it is preferred that the channels extend substantiallylinearly and parallel to one another (such as, for example, in the caseof an extruded honeycomb body), but this is not absolutely necessary.The channels and/or the channel walls can also be embodied withstructures (grooves, knobs, sliding surfaces, etc.) in order toimplement an additional improvement of the formation of contact betweenthe retained soot particles and the nitrogen dioxide from the exhaustgas. As a result, the structure extends from the channel wall into thechannels and, in particular, constricts the channel cross section. Thechannel walls may be impermeable and/or permeable to gas in this case,with it being possible to implement the latter by porosity of thematerial and/or by openings (for example holes).

In accordance with an added feature of the invention, variousrefinements of the at least one deposition inhibitor can be assigned tothe channels. That is to say in other words, in particular, that duringoperation the at least one deposition inhibitor results in differentdeposition regions of the individual channels being influencedchronologically and/or spatially. Deposition inhibitors can therefore beembodied differently in the region of the centrally disposed channelsthan the deposition inhibitors in the channels in the edge region of thesurface precipitator. Alternatively or cumulatively, it is possible toembody the deposition inhibitors near the inlet region differently thanin the region of the outlet region for a number of channels. If thesurface precipitator is itself a neutralization element, it is thereforepossible to implement different electrical conductivity of the channelwall in particular in the profile direction of the channels and/or as afunction of the position of the channels, as a result of which theembodiment of the electrical field and therefore the effect of theCoulomb's forces on the electrically charged soot particles is adapted.

In accordance with an additional feature of the invention, the at leastone deposition inhibitor extends in various axial zones of the channels.Consequently, in some of the channels, a deposition inhibitor may beembodied from the inlet region to several millimeters into a channel,but it is also possible for the deposition inhibitor to extend as far asthe outlet region. In this context, a plurality of deposition inhibitors(of the same type and/or different) can also be provided in therespective channels.

In accordance with yet another feature of the invention, the at leastone deposition inhibitor is formed by various channel forms or shapes.In this specific embodiment of the surface precipitator, in particularif the latter constitutes a neutralization element, it should be madepossible that the probability of impacting and/or the depositingcapability and/or the flow paths for soot particles be adapted to theflow profile of the exhaust gas and/or the exhaust gas volume flow. Thisis intended, in particular. to counteract a situation in which differentquantities of particles are made to flow into the channels. Instead, itis possible to implement a situation in which there are differing (andon average therefore uniform) flows into the channels given differentstreams of exhaust gas. The term “channel shape” therefore includesconsidering its cross section and the profile shape of the channel.Different channel shapes are provided, in particular, when the channelsdiffer in one of the following properties: size (diameter) of thechannel cross section, shape of the channel cross section, inclinationof the channel profile, curvature of the channel profile, widening ofthe channel profile, constriction of the channel profile, positionand/or type of the structures of the channel walls, etc.

In accordance with yet a further feature of the invention, channels inwhich there is a central flow are made at least larger or morestructured than channels in which there is an off-center flow. Thismeans, in other words, that relatively large channel cross sectionsand/or a smaller channel density is provided in the central region ofthe surface precipitator, and/or in the region thereof in which there isa central flow, than in the region of the surface precipitator which isoff-center or near the edge. Alternatively or cumulatively to this, itis possible for the channel walls of the channels in which there is acentral flow to be embodied to a relatively large degree withstructures, that is to say with respect to the size of the structures,the frequency of the structures, etc. This measure takes into account,in particular, the fact that the pressure loss of the exhaust gasincreases quadratically with the mass flow of exhaust gas as it flowsthrough the surface precipitator. The provision of structures in thecentral region then leads to a situation in which overproportionallylarge parts of the stream of exhaust gas are also conducted into theedge regions for this part, with the result that in total approximatelythe same flow can be detected in the channels.

In accordance with yet an added feature of the invention, the at leastone deposition inhibitor can be formed by at least one electricalinsulator. In particular, ceramic coating is possible for this purpose.The coating can also be applied, for example, on metal surfaces.Alternatively, separate components which form an electrical insulatorcan also be added.

In accordance with yet an additional feature of the invention, it isbasically also possible for the at least one surface precipitator tohave, as a basic material, an electrical insulator which forms adeposition inhibitor, that is to say in other words, that the surfaceprecipitator itself has at least partially no electrical conductivity.Then, in particular, by taking into account the channel shape (forexample twisted and/or structured channels) or the concept of thesurface precipitator (for example wall-flow filter with alternatelyclosed channels) it is possible to ensure that the particles can bedeposited on or in this basic material.

In accordance with again another feature of the invention, in order toalso achieve the selective desposition of particles in a uniform fashionin this case over the entire surface of the surface precipitator, it ispossible to integrate electrical conductivity, for example in such a waythat the surface precipitator has at least one neutralization element,wherein the at least one neutralization element is embodied differentlyin the channels. Consequently, electrically conductive materials can bedisposed in and/or on the basic material of the surface precipitator andare connected to an electrical ground. This can be achieved bycorrespondingly electrically conductive conductors, fibers, particlesand the like (which are, if appropriate, in contact with one another).In particular, metal inlays are suitable for this purpose.

The surface precipitator preferably has porous channel walls which areformed in particular, with sintered material, ceramic, silicon carbide(SiC) or mixtures thereof. This is particularly preferably an (at leastpartially) extruded honeycomb body.

With the objects of the invention in view, there is also provided amethod for converting soot particles of an exhaust gas. The methodcomprises:

-   -   a) providing at least nitrogen dioxide or oxygen in the exhaust        gas;    -   b) ionizing soot particles with an electrical field;    -   c) depositing electrically charged soot particles on inner        channel walls of at least one surface precipitator; and    -   d) placing at least nitrogen dioxide or oxygen in contact with        the soot particles deposited on the inner channel walls of the        at least one surface precipitator.

As a result, in particular continuous regeneration of soot particles isalso specified with the device proposed according to the invention. Forthis reason it is also necessary to point out that the features whichare presented with respect to the devices can be used to explain themethod, and vice-versa. It is also necessary to note that the steps a)and b) can be carried out in succession and/or simultaneously, whereinemphasis is placed on the respective effect on a partial stream ofexhaust gas. It is preferred that all of the steps a) to d) be carriedout continuously during the operation of a mobile internal combustionengine. Furthermore it is preferred that step a) alternatively includesthe provision/generation of nitrogen dioxide or oxygen within theexhaust line.

In accordance with another mode of the method of the invention, the flowbehavior of the exhaust gas through the channels of the at least onesurface precipitator or the configuration of the electrical field isvaried as a function of an exhaust gas parameter. If appropriate, it isalso possible to carry out both measures simultaneously or alongside oneanother. The flow behavior can be changed independently, for example bycorrespondingly configuring the surface precipitator with the resultthat there is a flow through different channels given differing flowbehavior. However, it is also possible to perform active variation inthat, for example, the frequency and/or field strength of the electricalfield are regulated. In particular, the temperature, the mass flow, thevolume flow and/or the flow speed of the exhaust gas are considered asexhaust gas parameters. Of course, other characteristic values of theoperation of the internal combustion engine can also be used as analternative thereto or additionally in order to draw conclusions about acorresponding exhaust gas parameter.

In accordance with a further mode of the invention, the method servespreferably to perform continuous regeneration of the at least onesurface precipitator. For this purpose, the soot is placed in contactwith nitrogen dioxide which has been generated upstream in the flow ofexhaust gas, and is chemically converted. For a further explanation,recourse can be made to the so-called CRT method (CRT=ContinuousRegeneration Trap) which is disclosed, for example, in European PatentApplication EP 0 341 832, corresponding to U.S. Pat. No. 4,902,487.

In accordance with a concomitant mode of the invention, a method is alsopreferred in which the electrically charged soot particles are depositeduniformly on all of the channels of the at least one surfaceprecipitator. This is to be understood, in particular, as meaning thatthe device and/or the surface precipitator is configured in such a waythat the probability of deposition of the electrically charged sootparticles on all of the inner channel faces of the surface precipitatoris substantially the same. The measures which are to be taken, ifappropriate, to achieve this have already been presented above.

The invention is preferably used in a motor vehicle having an internalcombustion engine, in particular a diesel engine, with a downstreamexhaust system which has a device according to the invention. For thispurpose, the motor vehicle has, for example, a control unit foroperating this device with the method according to the invention,wherein the control unit is configured, in particular through the use ofcorresponding software, to implement the operation according to theinvention during the operation of the motor vehicle. If appropriate, thecontrol unit can interact with sensors of the exhaust system and/or ofthe internal combustion engine and/or stored data models in order toadapt this system.

Other features which are considered as characteristic for the inventionare set forth in the appended claims, noting that the features which aredisclosed individually in the claims can be combined with one another inany desired technologically appropriate way and indicate furtherembodiments of the invention.

Although the invention is illustrated and described herein as embodiedin a method for treating exhaust gas containing soot particles, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagrammatic, longitudinal-sectional view of a firstembodiment variant of a device according to the invention;

FIG. 2 is an enlarged, fragmentary, longitudinal-sectional view of afirst embodiment variant of a surface precipitator;

FIG. 3 is a fragmentary, longitudinal-sectional view of a second variantof a surface precipitator;

FIG. 4 is a fragmentary, longitudinal-sectional view of a thirdembodiment variant of a surface precipitator;

FIG. 5 is a longitudinal-sectional view of a further embodiment variantof the device;

FIG. 6 is a diagram illustrating the method according to the invention;

FIG. 7 is an illustration of attraction forces acting on a sootparticle; and

FIG. 8 is a longitudinal-sectional view of a further embodiment variantof a surface precipitator.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a first exemplaryembodiment of a device 1 according to the invention. In this case,exhaust gas flows in a flow direction 31 through an exhaust system,which is illustrated herein in an approximately tubular shape, althoughthat is not significant. The exhaust gas contains soot particles 2. Theexhaust gas with the soot particles 2 is firstly conducted through anitrogen dioxide source 3, in particular through a honeycomb-shapedcatalytic converter with a platinum coating. This nitrogen dioxidesource converts nitrogen monoxides (NO) contained in the exhaust gasinto nitrogen dioxides (NO₂), as a result of which the proportion ofnitrogen dioxides in the exhaust gas is increased (making up, inparticular, at least 25% by weight or even at least 50% by weight of theentire nitrogen oxides). The exhaust gas which is prepared in this wayflows on to an (individual) surface precipitator or collector 6. Thesurface precipitator 6 has an inlet region 8 and an outlet region 9.Linear channels 7, which extend parallel to one another, run between theinlet region 8 and the outlet region 9. The channels 7 are embodied inthis case (partially) with a catalytic coating 18, but this is notabsolutely necessary.

Before the exhaust gas reaches the surface precipitator 6, it enters anelectrical field which is formed through the use of an ionizationelement 4 upstream of the surface precipitator 6 and a neutralizationelement 5 at the outlet region 9 of the surface precipitator 6. In theillustrated case, the neutralization element 5 is integrated into thechannel walls of the surface precipitator 6. In order to preventelectrically charged soot particles from impacting directly on the frontface of a honeycomb body in the vicinity of the inlet region 8, and sothat the channels 7 themselves no longer serve for the accumulation ofsoot particles, a deposition inhibitor 10 is formed in the vicinity ofthe inlet region 8. This deposition inhibitor 10 actually reduces orprevents accumulation there. Different refinements of this depositioninhibitor 10 are also presented with reference to the following figures.

FIG. 2 shows, for example, details of a surface precipitator having amultiplicity of channels 7 which are bounded by channel walls 17. As aresult, in particular, a so-called honeycomb body 11 is formed. In orderto prevent the soot particles from being deposited only in the inletregion 8 of the honeycomb body 11, an electrical insulator 15 (embodiedin the manner of a coating on the channel walls) is provided there as adeposition inhibitor. This figure also indicates that the embodiment ofthe deposition inhibitor or, as shown herein, of the electricalinsulator, can relate to different zones 12 of the channels 7. The zones12 can therefore differ from one another, in particular in terms oftheir extent and/or position.

FIG. 3 shows an embodiment variant of a honeycomb body 11 which isformed with conically and/or tapering/widening channels 7. While thechannel shape 13 in FIG. 2 is, for example, substantially round and isconstant over its length, the cross section in the case of the conicalchannel shape changes in its longitudinal direction. Due to the changedchannel cross sections, the flow can also be influenced in this caseand/or deposition of soot particles can also be achieved in the rearpart of the honeycomb body 11. An oxygen source 32 is also indicated inpurely schematic form upstream of this honeycomb body 11. It is possibleto integrate this oxygen source 32 into an exhaust system, for exampleinstead of the nitrogen dioxide source.

Furthermore, a refinement of the honeycomb body 11 in which the channelwalls 17 are embodied with a basic material which acts as an electricalinsulator 15, for example ceramic or silicon carbide, is shown therein.However, in order to nevertheless motivate a movement of the sootparticles to the channel walls 17 on the basis of Coulomb's forces, thechannel walls 17 (which can, if appropriate, also be porous) haveelectrical conductors 30, for example in the manner of a reinforcement,embedded fibers, etc. The attraction force from the channel wall 17 tothe soot particles therefore becomes stronger over the length of thechannels 7, and/or this attraction force is smaller in the inlet region8. This axially staggered conductivity can occur on a zone-by-zone basisin this case, with the result that in each case approximately the sameconductivity is provided over predefined zones 12, but the transitioncan also be stepless or continuous.

FIG. 4 illustrates further details of a honeycomb body 11 as a surfaceprecipitator, wherein the channels 7 have different channel shapes 13.In a center 24, that is to say in a region of the surface precipitator 6in which there is a central flow, the channel cross section 7 isrelatively large. If the channel shape 13 is considered in the directionof a radius 27, it is to be noted that the cross section of the channelsis smaller in the region of an edge 25, that is to say a region in whichthere is an off-center flow. In addition, it is noted that (only) thechannels 7 in the region of the center 24 have structures 14. Thesestructures build up a relatively large pressure drop, in particular asthe flow speed of the exhaust gas increases or the volume flow of theexhaust gas becomes larger, as a result of which the exhaust gas is alsoconducted to a greater extent in radially outer channels. These measurescontribute, in particular, to bringing about uniform loading with sootparticles and uniform provision of nitrogen dioxide for the depositedsoot particles.

FIG. 5 shows a further exemplary embodiment of the device 1 according tothe invention. A left-hand partial region of the figure illustratesagain how the exhaust gas containing soot particles 2 flows through thenitrogen dioxide source 3 in the flow direction 31, as a result of whichmore nitrogen dioxide is formed. In turn, an electrical field 16 isformed below, but this time through the use of an ionization electrode28 which serves as an ionization element 4 and a ground electrode 29which is disposed downstream of the surface precipitator 6 and serves asa neutralization element. Consequently, the surface precipitator 6 iscompletely located in the electrical field 16.

The surface precipitator 6 illustrated therein is, in particular, aconventional wall-flow filter made of ceramic or silicon carbide, thechannels of which are alternately closed, as a result of which in eachcase flow dead ends are formed. However, the channels 7 do not, asillustrated therein, have to extend parallel to a central axis 26 of thehoneycomb body. Alternately positioned stoppers or plugs 23, which areprovided for the closure, can constitute a corresponding depositioninhibitor for electrically charged soot particles or be embodied assuch. The channel walls are embodied in this case in a porous and/orgas-permeable fashion, with the result that the soot particles arefiltered out. If electrical conductivity is present in such a surfaceprecipitator 6, for example as a result of direct contact with theground electrode 29 and with a corresponding configuration of thehoneycomb body, a correspondingly selected conduction of the electricalcharge should also take place. For this purpose, it is proposed that thehoneycomb body be surrounded by a mat 21 which brings about a sufficientdistance 22 from the housing 19 in order to avoid a voltage rolloverfrom the surface precipitator 6 to the housing 19. If the honeycomb bodyis metallic and has its own casing 20, the same applies.

FIG. 6 is an illustration of individual method steps. In this case, in afirst step, nitrogen oxides (NO_(x)) and/or nitrogen monoxide (NO) isconverted into nitrogen dioxide (NO₂) through the use of the nitrogendioxide source (and/or a corresponding catalytic coating). Furthermore,the soot particles (PM) or some of the soot particles are ionized, as aresult of which they have a purely electrical charge. The electricallycharged soot particles (PM⁺) are then deposited uniformly on a channelwall with the aid of corresponding electrostatic attraction forces,which takes place very uniformly where possible. The soot particles(PM⁺/PM) which are spaced apart to a greater extent and are, ifappropriate, still electrically charged or even already neutralized, arefreely accessible to the generated nitrogen dioxide (NO₂), as a resultof which simple and effective regeneration of the deposition surfaceand/or of the filter material is made possible. Catalysts can also beused in supportive fashion for this conversion process. After theconversion of the soot particles, the gaseous residues such as, forexample, carbon dioxide (CO₂) and elementary nitrogen (N₂) are removedfrom the surface precipitator.

FIG. 7 is an exemplary and illustrative view of the effect of thesurface precipitator 6 on the soot particle 2. The soot particle 2accordingly flies, for example in the flow direction 31, through pores33 of the surface precipitator 6, while being electrically charged inthe process. Due to the potential toward the surface precipitator 6,this soot particle 2 does not fly linearly onward (as indicated bydashes) but instead experiences a deflection 34 and comes to bear on thesurface precipitator 6. The soot particle 2 can then be correspondinglyconverted there.

FIG. 8 shows details of a further embodiment variant of a surfaceprecipitator according to the invention with a multiplicity of channels7 which are bounded by channel walls 17. As a result, in particular, aso-called honeycomb body 11 is formed. The honeycomb body 11 is formedfrom an insulating material, preferably ceramic. In order to bring abouta preferred deposition of soot particles in the honeycomb body 11, evendownstream of the inlet region 8 of the honeycomb body 11, electricalconductors 30, which extend to different degrees in the direction of theinlet region 8 in different zones 12 of the honeycomb body, are providedin the honeycomb body which is embodied as an electrical insulator 15.

The invention provides, in particular, uniform deposition of the sootparticles and continuous regeneration of the surface precipitator.

The invention claimed is:
 1. A method for converting soot particles ofan exhaust gas, the method comprising the following steps: providing adevice for treating the soot particles having: at least one source ofnitrogen dioxide or oxygen; at least one ionization element for ionizingsoot particles; at least one surface precipitator having an inletregion, an outlet region and a plurality of channels through which theexhaust gas can flow, the channels extending between the inlet regionand the outlet region; and at least one deposition inhibitor disposed atleast partially at the inlet region for inhibiting deposition ofelectrically charged soot particles; a) providing at least nitrogendioxide or oxygen in the exhaust gas by using the at least one source ofnitrogen dioxide or oxygen; b) ionizing soot particles with anelectrical field created by the at least one ionization element; c)depositing electrically charged soot particles on inner channel walls ofthe channels of the at least one surface precipitator and preventing apredominant deposition of electrically charged soot particles in theinlet region by using the at least one deposition inhibitor; and d)placing at least nitrogen dioxide or oxygen in contact with the sootparticles deposited on the inner channel walls of the at least onesurface precipitator.
 2. The method according to claim 1, which furthercomprises carrying out steps a) and b) in succession.
 3. The methodaccording to claim 1, which further comprises carrying out steps a) andb) simultaneously.
 4. The method according to claim 1, which furthercomprises carrying out steps a) to d) continuously during operation of amobile internal combustion engine.
 5. The method according to claim 1,which further comprises providing or generating nitrogen dioxide oroxygen within an exhaust line in step a).
 6. The method according toclaim 1, which further comprises varying a flow behavior of the exhaustgas through the channels of the at least one surface precipitator as afunction of an exhaust gas parameter.
 7. The method according to claim1, which further comprises varying a configuration of the electricalfield as a function of an exhaust gas parameter.
 8. The method accordingto claim 7, which further comprises varying at least one of a frequencyor a field strength of the electrical field.
 9. The method according toclaim 6, which further comprises using at least one of a temperature, amass flow, a volume flow or a flow speed of the exhaust gas as theexhaust gas parameter.
 10. The method according to claim 7, whichfurther comprises using at least one of a temperature, a mass flow, avolume flow or a flow speed of the exhaust gas as the exhaust gasparameter.
 11. The method according to claim 1, which further comprisescontinuously regenerating the at least one surface precipitator.
 12. Themethod according to claim 1, which further comprises uniformlydepositing the electrically charged soot particles on all of thechannels of the at least one surface precipitator.
 13. A method ofconverting soot particles of an exhaust gas in an exhaust system of amotor vehicle, the method comprising the following steps: a) providingat least nitrogen dioxide or oxygen in the exhaust gas by using at leastone source of nitrogen dioxide or oxygen; b) ionizing soot particleswith an electrical field created by at least one ionization element forionizing soot particles; c) depositing electrically charged sootparticles on inner channel walls of channels of at least one surfaceprecipitator, and preventing a predominant deposition of electricallycharged soot particles in an inlet region of the at least one surfaceprecipitator by using at least one deposition inhibitor disposed atleast partially at the inlet region; and d) placing at least nitrogendioxide or oxygen in contact with the soot particles deposited on theinner channel walls of the at least one surface precipitator.